This invention pertains to a method of operating a steam turbine using turbine reserve capacity for operation at elevated loads.
Large steam turbines of the type used in the electrical power generating industry are liberally designed to provide some additional load capability beyond the nominal rated capacity, an operating point commonly referred to as the "guarantee point." The nominal rated capacity is stated in terms of power output, and conventionally, this condition is achieved with the control valves less than fully open so that the additional capability is obtained by opening the control valves fully. If the turbine design is such that the nominal rated capacity occurs with the steam admission valves fully open, the turbine efficiency at that point will be improved significantly in terms of energy utilization or heat rate. However, with the control valves fully open, there are limited means by which reserve capacity of a steam turbine can be achieved.
One known method of achieving excess capacity in a turbine when the nominal rated capacity occurs with the control valves fully open, is to provide a bypass valve and thereby pass extra steam around the control valves to a latter lower pressure stage of the turbine. This method (as used in the past) has three disadvantages. First, it has been considered necessary to integrate the bypass valve into the turbine control system, in effect making the bypass valve an additional control valve which is throttled in a controlled and coordinated manner with the admission control valves. This adds significantly to the complexity of the control system. Second, to meet industry incremental regulation requirements with a throttling type bypass valve, it has been necessary to provide some overlap between the control valves and the bypass valve. In other words, it becomes necessary to begin opening the bypass valve before the control valves are fully open. This degrades the efficiency of the turbine at its nominal rated capacity. Third, because of the small capacity of such a bypass valve, considerable valve stroking motion is required to have the turbine participate in frequency control in the power system to which it is connected. This large motion may cause heavy wear and lead to early failure of the valve.
Accordingly, it is an object of the present invention to provide a method for operating a steam turbine by which a bypass overload valve is used to achieve reserve capacity of the turbine with no substantial change to the turbine control system and in which a throttling type bypass valve is not required to be used.
Further, it is an object of the invention to maximize steam turbine efficiency at the nominal rating point by the use of a bypass overload valve while eliminating the need to provide overlap in operation between the control valves and the bypass overload valve.